Liquid crystal display devices are electronic devices for converting electrical signals generated from various devices into visual information by using variations in the transmissivity of liquid crystals caused by a voltage applied thereto.
Owing to merits such as small size, lightness, and low power consumption, liquid crystal display devices have been prominent as replacements for cathode ray tubes (CRTs) widely used in the related art, which are capable of overcoming disadvantages of the CRTs, and currently, they have been mounted in almost all of information processing apparatuses requiring display devices.
In such a liquid crystal display device, a voltage is applied to liquid crystals to change the molecular arrangement thereof and thus convert changes in optical characteristics of the liquid crystals such as birefringence, optical rotatory power, dichroism, light scattering or the like into visual changes. That is, liquid crystal display devices may modulate light using liquid crystals.
Since liquid crystal display devices do not produce light themselves, additional light sources are necessary to illuminate liquid crystal device screens. Such illumination light sources are generally called backlight units.
In general, backlight units may be classified as edge-type backlight units and direct-type backlight units, according to the position of a light emitting lamp. Edge-type backlight units include a lamp at a side of a light guide panel that guides light emitted from the lamp. Edge-type backlight units are used in small liquid crystal display devices such as desktop computer monitors and laptop computers and are advantageous in terms of evenness of lighting, durability, and size reduction. On the other hand, direct-type backlight units are developed for application in display devices having 20-inch or larger screens and may directly illuminate the entire surface of a liquid panel by having a plurality of lamp light sources arranged under the liquid crystal panel.
Linear light sources such as cold cathode fluorescent lamps (CCFLs) were previously widely used as lamps for backlight units. However, light emitting diodes (LEDs) are now widely used as lamps for backlight units due to merits thereof, such as good color reproductivity, environmental-friendliness, slimness and lightness, and low power consumption.
Meanwhile, a plurality of optical films may be variously combined and used in a backlight unit according to the related art for the purpose of diffusing or concentrating light generated from a light source, improving brightness, or reducing lamp mura.
Recently, brightness and a viewing angle have become very important properties in image display devices, such as an LCD device, and the properties are known to be determined by optical sheet properties. Recently, hemispherical micro lens array sheets have been widely used; however, hemispherical micro lens array sheets have limitations in terms of an increase in luminance and defects, in that luminosity thereof is relatively deteriorated in the case of improving the viewing angle.
Currently, research and development aimed at the slimming and lightning of a backlight unit is ongoing, and in particular, a micro lens array sheet having improved luminance and viewing angle characteristics and a backlight unit including the same are required.